Tuned band-pass crystal holder



July 28; 1959 w, C, FARMER 2,897,364

TUNEJD BAND-PASS CRYSTAL HOLDER lFiled NOV. 9, 1955 ATTORNEYS TUNE!) BAND-PASS CRYSTAL HOLDER William C. Farmer, Baltimore, Md., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application November 9, 1955, Serial No. 546,015 1 Claim. (Cl. 250-31) The present invention relates to a tuned bandpass crystal holder and more particularly to a tuned bandpass crystal holder employing cavity resonators.

Ultra high frequency crystal video receivers require Wide band frequency reception while high rejection is necessary for frequencies not in the pass band. Crystal holders to date are of two types: the series coaxial line type holder with and without stub matching and the high Q resonant type holder. The coaxial type has very little selectivity, and the resonant type has a very narrow band.

The present invention is a tuned band-pass crystal holder using two or more cavities with tuned coupling loops between cavities. The crystal is received into the last cavity in a manner such that it makes contact with the center conductor. The iirst cavity is a high Q cavity, but due to the heavy loading of the crystal, the Q of the last cavity is much lower. Since the effective loaded Q of two or more tuned circuits tuned to the same frequency is the root of the product of the Qs of each individual circuit, the eiective Q of the present crystal holder can be made suiciently low that the holder will pass a wide band, but due to resonant characteristics the bandpass curve will have steep slopes thereby giving considerable selectivity. Also, desired preselection may be obtained in a minimum space and weight and wide band selectivity may be attained with little or no insertion loss over the pass band.

Accordingly, an object of the present invention is the provision of a crystal holder with which wide 4band selectivity may be attained with little or no insertion loss over the pass band.

Another object is to provide a crystal holder which can give broad band response and also reasonable selectivity.

A further object of the invention is the provision of a crystal holder with `which desired preselection may be attained in a minimum space and with minimum weight.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. l shows a section of a preferred embodiment of the invention taken on the line 1-1 of Fig. 2 looking in the direction of the arrows,

Fig. 2 illustrates a top view of the device, and

Fig. 3 shows a section of the device taken on the line 3-3 of Fig. 2 looking in the direction of the arrows.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. l (which illustrates a preferred embodiment) an input jack body 11 and insert 13 for connection to the input coaxial line, and probe 15 which capacitively couples the input signal into the first cavity 17. The first cavity 17 and also the second cavity 19 are formed from a conductive base 21 and top plate 23, the latter of which is detachable by rates Patent O means of screws 25. Stub 27 is provided to tune cavity 17 and stub 29 to tune cavity 19, and coupling between these two cavities is obtained by means of adjustable coupling loop 31.

In the top view illustrated in Fig. 2, crystal holder base plate 33 is shown attached to base 21 by means of screws 35. A crystal holder cap 37 is threaded on plate 33 and in cooperation with crystal output connection 39 retains crystal 41 in spring clip 43 which is in electrical Contact with the stub 29. From this gure it can be seen that cavity 19 is an open circuit tapped coaxial line; i.e., the cavity is a quarter-wave resonate line as compared with a shorted line which is a half wave length cavity and the crystal is connected to the center conductor, consequently the line is tapped.

More of the detail of the structure immediately surrounding crystal 41 is shown in the section of Fig. 3.

A conductive sleeve 45 encloses a portion of the crystal 41, and is one plate of a RF. (radio frequency) bypass capacitor whose dielectric is insulating sleeve 47 and insulating disk 49, `and whose other plate comprises plate 33 and base 21. It is not necessary that this capacity be part of the cavity structure; however, it makes a short R.F. bypass. An insulating sleeve 51 is provided to insulate cap 37 from connection 39 so that these elements can connect this crystal holder to an output line.

Although the input coupling to cavity 17 is shown to be capacitive, other types may be used. Basic reasons for employing capacitive input are simplicity of manufacture and ease of adjustment. By adjusting probe 15, the proper Q can be maintained on all crystal holders manufactured. Direct (input connected directly to the center conductor of the first cavity) and inductive loop coupling can be used quite effectively. The direct and loop coupling are, of course, essentially current coupling devices and would be inserted or connected near the top plate (the high current end of the cavity). Also it is to be realized that though only two cavities are shown, the actual num-ber of cavities employed will be dependent upon the specific application of the crystal holder and in some applications may be three or more.

The eifective loaded Q of two or more tuned circuits tuned to the same frequency is the root of the product of the Qs of each individual circuit; i.e.,

where Qe is the effective Q, and n is the total number of circuits. Thus, the first cavity can have a high Q which would give much selectivity but little bandwidth, but if the other cavities have low Qs, the cumulative effect of all the cavities can be a medium or low Q and thus a broader band width can be obtained.

The cavity tuning procedure for the illustrated arrangement is as follows: A signal is applied at input insert 13 and jack body 11 to probe 15, and a measuring device is connected via holder cap 37 and output connection 39 to the output of crystal 41. The cavities 17 and 19 are decoupled by sliding the coupling loop 31 in until shorted against base 2l. Cavities 17 and 19 are tuned for maximum output by adjusting stubs 27 and 29, respectively. The position of coupling loop 31 is then adjusted for maximum output and the tuning procedure is completed.

This crystal holder is not limited to detection of the input signal but may be used as a mixer by insertion of a second signal into cavity 19. Other uses will suggest themselves.

Obviously many modications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within A crystal holder' comprising: input coaxial line means f for connection to a source of input signal, a high Q resonant cavity, a low Q resonant cavity 'which is an open circuit tapped coaxial line, said cavities being formed in` a conductive base, said low Q resonant cavity having a center conductor and an outer conductor, means for individually tuning said high Q and low Q cavities, input coupling means for coupling a signal from said input coaxial line means to said high Q resonant cavity, crystal retaining means for loading said low Q resonant cavity and directly connecting one terminal of a crystal to the center conductor of said low Q cavity, output means for connecting the other terminal of said crystal and the outer conductor of said low Q cavity to an output circuit, and adjustable loop coupling means having a rst position for decoupling Aand a second position for the maximum coupling of said low Q cavity and said high Q cavity, said loop coupling means in said first position being in contact with the base of said holder to thereby decouple said cavities, means for tuning said cavities for maximum output when said loop coupling means is in said first position, said loop coupling means being adjustably moved to said second position to provide maximum coupling Without returning said cavities, whereby wide band selectivity is obtained with practically no insertion loss over a given pass band.

References Cited inthe tile of this patent UNITED STATES PATENTS 2,455,657v Cork Dec. 7, 1948 2,513,761 Tyson July 4, 1950 2,550,409 Fernsl'er Apr. 24, 1951 2,580,678 Hansen et al. Ian. 1, 1952 2,587,055 Marshall Feb. 26, 1952 2,637,813' Braden May 5, 1953 2,638,544 Schreiner May 12', 1953 2,642,494 Zaslavsky et al. July 1 6, 1953 

